Ku'Gath is too expensive. His pie plate looks nice on paper, but it only wounds on a 4+ and most enemies will have cover too, so you'll be killing 25% of what you hit. Not worth twice the cost of a GUO. Grab a normal GUO w/ Breath and give your DPs wings. Nurglings wouldn't hurt either.

Wings are too expensive, if you ask me. I'd run epidemius w/ a unit of 20 plaguebearers. with ku'gath out throw in 3 suicide breath nurgle heralds. If you drop those with breath princes, 1st turn tally baby! If points becom an issue drop a beast or two, beasts are only good once the tally is up. Just a thought.

I have been running a mono-nurgle demon list since release of demons and the suggestions I would make would be:

I would disagree, Ku'gath has +1 attack +1 wound necrotic missile and nurgling infestation is worth the 105 more points you will pay,which isn't double the cost of a normal GUO after you buy the abilities you would for a GUO. The necrotic missile may only wound on 4+ but its still another shooting attack he can use and take advantage of being a MC and shooting two weapons. I had kugath take down 2 dakka tyrants and 6 tyrant guards w/lash whips and a dakkafex over 3 turns, my opponent underestimated his survivability when he assaulted him in some trees. Outside of KP missions the infestation is good especially when tally starts wracking up. Though you could get a normal GUO and a Herald w/palanquin,boc,cof for ku'gath. Also the necrotic missile is the good for its range.

I run aura of decay on my dps and unless there are t3 models its nearly worthless, though very good against hordes.
I would consider dropping the beasts and getting 2x6 units of nurglings and put instruments on your plaguebearers, though instruments not needed they are pretty decent upgrade for the unit.

I have found the wings work pretty well with the DPs and when I don't use them wish I had even at how costly they are, they are the fastest unit Nurgle can get.

Like alot of demon lists but even more so mono nurgle lacks anti tank so I have recently considered putting in a unit of 3xscreamers of T for the ability to handle LRs.